This invention relates to electrooptic modulation of a light beam to produce a line array of data suitable for imprinting pixels of an image on a print medium. More particularly, the invention provides a laminar assembly of comb electrode layers interleaved among layers of electrooptic film material wherein reduced interelectrode spacing lowers required voltage, and multiple arrays of electrodes produce an adequately long optical interaction region along an optical path.
Electrooptic modulators have been employed for modulating beams of light. One form of modulator of considerable interest employs a line array of electrodes in the form of a comb electrode structure wherein sequential ones of the electrodes are provided with voltages from a suitable source of signal voltages to induce corresponding electric fields within the electrooptic material. A beam of light, incident upon the electrooptic material passes first through a polarizer which orients the electric vector of the incident light in a direction inclined at 45 degrees relative to the direction of the comb electrodes. In the construction of the modulator, the electrodes are spaced apart sufficiently to provide for a fringing field which extends in a plane transverse to the electrodes, the plane containing an optical path along which the light propagates. The term "light", as used herein, includes not only the visible portion of the spectrum, but includes also infrared and ultraviolet portions of the spectrum in those situations wherein the polarizing material and the electrooptic material is responsive to these portions of the electromagnetic spectrum.
Light propagating through a region of the electrooptic material activated by the electric field experiences a change of polarization which develops in the following manner. The initially polarized wave can be regarded as having a component of electric field oriented perpendicularly to the electrodes, and a second component of electric field which is oriented parallel to the electrodes. In the absence of the applied electric field, both components of the incident optical signal propagate with the same speed of propagation along the optical path through the electrooptic material. However, in the presence of the applied electric field in the plane transverse to the electrodes, the component of the optical signal having its electric field in the transverse direction experiences a reduction in propagation speed relative to the portion of the optical signal having its electric field parallel to the electrodes. As the two components of the optical signal propagate through the interaction region, the differences in speeds of propagation introduce a continually increasing phase shift between the two optical components.
The amount of phase shift experienced depends on the length of the interaction region, the wavelength of the light in the material, and upon the strength of the applied electric field up to a maximum value wherein the electrooptic effect saturates. The length of the interaction region and the magnitude of the applied electric field may be selected to produce a relative phase shift of 180 degrees between the two components of the optical signal in propagation through the interaction region. The result is a polarization of resultant optical signal which is perpendicular to that of the incident polarization. A second polarizing unit, or analyzer, is positioned at the output side of the electrooptic material, and is oriented at 90 degrees relative to that of the input polarization. Thereby, the outputted signal propagates through the analyzer in the presence of the applied transverse electric field to the electrodes. In the absence of the applied electric field, the exiting optical signal has a polarization perpendicular to that of the analyzer, and is blocked by the analyzer. Alternatively, it will be well understood that the analyzer may be parallel to the polarizer in which case the modulator will be light transmissive "open" when unexcited and light blocking "closed" when excited.
It has been found useful to construct light modulation devices with electrooptic modulators of the foregoing form. By activating successive pairs or groups of electrodes to establish a presence or absence of electric field independently for each of the pairs or groups of electrodes, it is possible to establish a line array of pixels wherein individual ones of the pixels are characterized by the presence or absence of light exiting the modulator.
A problem arises in the use of the foregoing arrangement of electrodes for directing the exiting light of the modulator upon a print medium, so that print markings appear on the print medium in response to the presence and absence of light of the various pixels which are to be printed. The effective length of the interaction region is related directly to the spacing between the electrodes. This is apparent from the geometry of the fringing electric field. In the case of a comb electric structure disposed directly upon the surface of a block of electrooptic material, the useful portion of the fringing field extends into the electrooptic material a distance approximately equal to the interelectrode spacing. In order to ensure an adequate strength of electric field throughout the interaction region, the electrodes must have a correspondingly large signal voltage of hundreds of volts. Such large voltage is inconvenient to generate by signal generators, requires specialized circuitry, and may cause electrical breakdown between electrodes. A further disadvantage is the relatively large size of the electrode array, particularly with respect to the size of pixels which are to be developed on a print medium.